This section provides background information related to the present disclosure which is not necessarily prior art. This section also provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
Cold spraying is a type of additive process in which a stream of solid particles is accelerated to high speeds by a carrier gas through a nozzle toward a substrate. The particles have enough kinetic energy such that upon impact with the substrate, they deform plastically and bond metallurgically/mechanically to the substrate to form a coating. Although, metallurgical bonding is preferred, all the particles may not be necessarily bonded in a metallurgical fashion.
The particles are accelerated to a critical velocity such that the coating can be created. This critical velocity can depend on the properties of the particles and to a lesser degree on the material of the substrate (i.e., deformability, shape, size, temperature, etc.).
It is hypothesized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and strain (mechanical) energy leading to a phenomenon known in art as “adiabatic shear instability.” It has been observed that the deposition efficiency of a given material is increased as the temperature of the particles is increased up to a certain extent, which is typically achieved by increasing the carrier gas temperature. The carrier gas temperature also influences the gas dynamics through the convergent-divergent nozzle that is typically used in a cold spray process. In other words, all things remaining constant, a higher carrier gas temperature leads to a higher gas velocity in the divergent section of the nozzle, which in turn may lead to higher particle velocity.
Related U.S. application Ser. No. 12/959,523, taught a method for increasing the particle temperature in the divergent section of the nozzle (independent of the carrier gas temperature), by deploying a laser beam coaxially to the nozzle where the taught rectangular nozzle design along with the particle feeding methodology in the divergent section, enabled enhanced distribution and interaction of the particles with the laser beam within the divergent section of the nozzle to increase the temperature of the particles. In other words, the teachings of the referenced related art provided for a mechanism to independently control the particle temperature and the gas dynamics (velocity) for a given nozzle. Further, a portion of the laser beam also is transmitted to the substrate to enhance the deposition quality.
To deposit a coating, the cold spray nozzle is typically traversed on a substrate while maintaining a suitable target distance. This results in a coating along a small track (typically similar to the width of the nozzle exit) on the substrate. To coat a substrate having an area larger than the nozzle exit width, the nozzle is scanned on the substrate multiple times, typically in a raster pattern with the help of a motion system such as a robot. The nozzle exit width is limited by the gas dynamics requirements for a given convergent-divergent nozzle, to achieve the desired particle velocity as well as the distribution. In other words, for a given gas with a given inlet temperature and pressure, the geometry of the convergent section, the divergent section as well as the throat connecting the two sections, determine the gas flow behavior, which in turn influences the particle velocity and the particle distribution. It is not straight forward to just increase the nozzle cross section area to coat a larger substrate. For practical applications, it is recommended to optimize the geometry of the nozzle so that the necessary flow dynamics can be achieved with standard industrial equipment (gas supply, heater, powder feeder etc.) economically.
One particular difficulty associated with cold spray process arises from defects on the underlying substrate surface. When the surface has an imperfection such as a gap or undulation between two coating passes (tracks), the discontinuity/imperfection may continue to develop in the subsequent layers as the coating builds up. Therefore, it is not recommended to build a thicker layer in a single pass, which may become the precursor to the undulations in the final coating. Further, while coating circular/conical objects with varying surface area, extensive process optimization is required to manage the deposit thickness from the origin towards the periphery or from the vertex towards the base, as the coating mass changes significantly.
From the aforementioned, it is apparent that uniform coatings on large substrates with varying surface area by a single cold spray nozzle, is not easily achieved. New methods and devices are needed for efficient coating fabrication.
The following summary is provided to facilitate an understanding of some of the innovative features unique to the present disclosure and is not intended to be a full description. A full appreciation of the various aspects of the disclosure can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is desirable that a cold spray apparatus is capable of coating complex large substrates with high accuracy and efficiency, without necessitating complex tool path optimization and in turn eliminating/minimizing defect growth and subsequent finish machining operation.
Provided is a cold spray apparatus for applying a coating of particles to a substrate, comprising a plurality of nozzles, including each nozzle defining an inner passage that terminates at a common exit for the entire assembly. The nozzle assembly also includes a particle supply members in communication with the inner passages. The particle supply members supply the particles to flow and accelerate through the inner passages and out of the nozzle via the common nozzle exit toward the substrate to be coated thereon. Furthermore, each nozzle includes a laser beam that is transmitted through the inner passage and exits via the common nozzle exit toward the substrate. The laser heats at least one of the particles within the inner passage and the substrate to promote coating of the substrate with the particles.
Provided are methods that solve one or more problems of the prior art optionally by providing in at least one aspect a method for enhancing the interaction of the particles with the laser beam within the inner passage of the nozzle, and thereby improving the energy absorption. This includes minimizing backward scattering of the laser beam by injecting particles in the divergent section of the nozzle, distributing the particles uniformly therein and hence increasing the interaction of the particles with the laser beam.
In some aspects, methods for integrating the particle stream from each nozzle into a common particle stream having substantially uniform particle distribution density and directing the combined stream with substantially uniform particle characteristics towards the substrate are provided to increase the deposition efficiency and uniformity. This optionally includes terminating each nozzle's inner passage at an optimal distance from the common exit of the apparatus assembly.
In yet another aspect, methods for coating complex substrates is provided. This optionally includes organizing a plurality of nozzles, having a predetermined common exit geometry that mimics the substrate geometric profile to be coated or built. Yet further, this also optionally includes supplying a desired amount of particles to each nozzle to achieve differential coating mass on the substrate, which in turn develops a desired geometric profile or conformality.
Accordingly, it becomes possible to solve the above aforementioned problems and to coat complex substrates with high accuracy and efficiency.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.